A dual-channel, differential, high-speed, stopped-flow calorimeter has been constructed to study the effects of mixing speeds, flow velocities, and pressure drops on the thermal and optical time course of chemical reactions. The system uses a variable-speed flywheel and an electrically actuated clutch to transfer sufficient energy to the four drive syringes to mix reagent volumes of 200 microliters in 5 milliseconds or less. The inlet tubes from the drive syringes, as well as the mixer and the detection chamber, are kept at constant temperature between 25 degrees and 50 degrees C. An ultrafast thermistor (1 ms) is located in the detection chamber to measure the temperature rise during the reaction. The drive cam profile produces a constant acceleration of the syringes. An optically transparent mixing chamber allows optical measurement of the reaction. The observation tube is now made of cast urethane that has been coated with a 1000-Angstrom-thick layer of tantalum to prevent water vapor migration. The observation tube "floats" in O-ring seals to reduce shock loads during mixing. A linear optical encoder is used to measure the displacement of the drive syringes.